Steam generator



1957 w. L. HARDING ET AL 2,817,499

STEAM GENERATOR 2 Sheets-Sheet 1 Filed March 3, 1955 INVENTORQ Walter L. Hurdmg BY William A. Reed ATTORNEY Dec. 24, 1957 w. L. HARDING ET AL STEAM GENERATOR Filed March 3, 1955 2 Sheets-Sheet 2 INVENTORQ Walter L. Hardmg William A. Reed ATTORNEY United States atet-rt STEAM GENERATOR Walter L. Harding, Pittsburgh, Pa., and William A. Reed, Massapequa, N. Y., assignors to Combustion Engineering,- Inc.,' New-York, N. Y., a corporation of Delaware Application March 3, 1955, Serial No. 492,004

11 Claims. (Cl. 257-229) coils with the two fluids having characteristics such that r it is imperative that thefluidsbeprevented from coming into'direct contact with each other.

One such heat exchanger of this type is the steam generator-which employs sodium as the heating medium with the liquid sodium preferably being passed through the tubular elements while water is conveyed through the vessel and over the exterior of these'elements with the water absorbing heat from the sodium through the wall of these elements so that at-leasta portion of this water is convertedtosteam. Since his imperative that the sodium and water nevercome into direct contact the tubulareler'nents may be of double walled construction to provide the necessary safety factor required in this regard.

This type of heat exchangervpresents extremely difficult design problems which,'in certain-instances, are made evenmore difiicult by the requirement that the vessel have a longitudinal length as small as 'praoti'cable while at the same time be capableof housing the necessary 'amount ofw'heat exchange :'surface for "performance of the required operations.

One instance wherewthis requirementis essentialancl which is becoming increasingly. important *is the utilization of suchheat exchangers aboard ships where space isalways at a premium.

Wh'ile it may be essential that the-longitudinal 'diin'ension' of the vesselbe held within aspecific minimum dimension and at the same time have the vessel housefthe required number of square feet of heat transfer surface it is also essential that the diameter of the tu'bes thatare positioned within the vessel be as small as economic considerations-will permit with the pressure drop of the fluid flowing through the tubes being within tolerable limitsan'd it -is likewise essential that there beno excessivestresses developed due to differential thermal expansion with the 'juncture of the inner and outer walls of the double walled tubes beinga particular problem in this regard. #With the organization of the present invention'the requirements necessary for compliance with'the various factors are fulfilled in a novel and expeditious manner.

In accordance with the present invention'there is provided-a vertically disposed vessel preferably of cylindrical transverse sectionand within which is positioned a tube bundle which comprises-spiral tubular coils placed one' above the other and being made upof a pair of U- tubes disposed in sideby side relation and wound in a spiral with the return bend at the center of the spiral and withthe ends extending radiallythrough the vessel'wall in a'fluid tight manner. Each of the tubes is ofdouble walled construction with the inner tube being concentrically disposed within an outer tube so as to form an annular chamber therebetween with the inner and outer tubes being interconnected in a fluid tight manner at their ends. This annular space is filled with a fluid which has a high heat transfer coetficient, such as mercury, and a monitoring device is connected with this annular space so as to provide an indication if either the inner or outer tube is ruptured. Aheating fluid, such as liquid sodium, is-distributed to thetubes in the various coils by means of a-suitable manifold such asaninlet header connected with the tubes by short connecting spurs and after flowing through these tubular elements this fiuidl is collected in a manifold which may be substantially identical with the distributingmanifold and thereafter conveyed to a desiredpoint of use. During passage of this heating fluid through 'these tubular elements a portion of the heat contained thereinis transferred through the two walls which make upeach of these elements and the fluid disposed in the space between these two walls to the fluid, which may be water, that ispassed through the vessel and over the exterior ofthese elements withthis fluid preferably enteringthe lower end of the vessel and leaving the upper endof the vessel so that in acase of a steam generator a naturalcirculation through the vessel may be established. With this organization the diameter of the tube elements employed and the length of each tubular element may be such that the pressure drop of the fluid flowing therethrough is maintained well within permissible limits and at the same time there are no long straight runs of the double walled tubing which would cause excessive thermal stresses at the juncture of the two walls of the tubing, with the spiral configuration completely overcoming all difficulties in connection with excessive stresses due to differential thermal expansion.

lnorder that the tubular coil maybe readily drainable, withthis being important when a heating fluid such as sodium is passed through the tubular elements thereof, the coils are sloped gradually upwards to their center wherebythe tubular elements may be completely drained through the manifolds with which they are associated.

It i's an object of the present invention to provide an improved heat exchanger for'the indirect transfer or heat from ai -heating fluidto a fluid tobe heated.

Other and further objects of theinven'tion willbe'cor'ne apparent to those skilled-in the art as the description proceeds.

'With'the aforementioned objectsin view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner 'asto attain the results desired as hereinafter more particularly setforth in the following detailed description of an illustrative embodiment, said embodirhent being shown by the accompanying drawing wherein Fig. 1 is a verticalsectional view of the heat exchanger of the present invention showing the disposition of the tube bundle within the vessel andthe locationof one of the headers outside of the vessel.

Fig/2 is a transversesectionalview taken along line 2-2; of Fig. land showing the spiral formation ofthe coils sane tubebundle andthe'interconnection of the coils withthe' vertically disposed headers.

Fig. 3 is a fragmentary sectional view through one of the double walled tubular elements at the location of the projection of the elements through the vessel wall and showing the connection thereof with the vessel-wall andthe interconnection of the inner and outer tubes or Wall elements.

While the invention will be described as a steam gen erator withliquid sodium employed as the beating medium and with the sodium passing through the tubular coils and the water passing through the vessel and over the coils it is to be understood that this is merely illustrative and not restrictive and that the heat exchanger may be employed for uses other than the generation of steam and that the heating fluid may flow through the vessel and over the tubular coils rather than through the coils with the fluid to be heated then flowing through the coils rather than over the same.

Referring now to the drawing wherein like reference characters are used throughout to designate like elements the preferred embodiment of the present invention illustratively disclosed therein comprises the vertically disposed cylindrical vessel provided with openings 12 and 14 in its lower and upper ends, respectively, for the admission of a fluid such as water into the vessel and the discharge of this fluid, which may be converted to a steam and water mixture, from the vessel.

Mounted within vessel 10 is the tube bundle generally designated 16 and which is composed of a number of spiral tubular coils 18 (there being 24 such coils in the illustrative organization) positioned one on top of the other with the axis of each of the coils being parallel with and slightly ofi'set from the axis of the vessel 10. The coils are retained in a bundle and secured within vessel 10 by means of a suitable framework which includes the radially disposed spacers 20 which are positioned between each of the coils as shown and which have their outer ends welded to the longitudinally extending bars 22 that are in turn secured to the wall of the vessel. The radial inner and outer end of each of the spacers 20 are bored for the reception of tie bolts 24 and the spacers are in vertical alignment in four rows, as shown in Figs. 1 and 2, so that each of the tie bolts may pass through the bores in a single row with the tie bolts thus being eflective to secure and bind the coil in an integral bundle.

Each of the tubular coils consists of a pair of internested U-tubes positioned in side by side relation and wound into a spiral coil with the return bend of the U- tubes at the center of the coil and with the ends of the U-tubes extending radially through the wall of the vessel. With reference to Fig. 2, the top coil of the unit, which is shown in plan in this figure, is made up of the U-tube 26 which has legs 28 and 30 and the U-tube 32 which has legs 34 and 36.

In order that the tubes of the coils may be readily drained the coils are sloped upwardly toward the center in the form of a shallow cone providing for gravity flow out of the two ends of the tubes making up the coils.

In order to positively prevent direct contact between the heating medium that flows through the coils 18 and the medium to be heated, which is passed through the cylinder 10, the tubes of the coils are double walled, as shown in Fig.3, and are made up of a pair of concentric tubular elements 38 and 40 with the inner element 30 being sufficiently small to form a narrow annular passage 42 between these elements and which is filled with mercury or some other fluid having a high coefficient of heat conductivity. These two tubular elements are welded together at their ends in a fluid tight manner and for this purpose collar 44 is positioned over tubular element 38 and secured thereto by weld 46 with the inner end of this collar being received Within the expanded end of tubular element 40 and secured thereto by weld 48.

A detecting device of any suitable variety is associated with annular passage 42 for detecting the development of leaks which may be caused by the rupture of either of the tubular elements 38 or 40 and one such detecting device is shown in Fig. 3 and merely comprises expansion chamber 50 interconnected with passage 42 through conduit 52 with pressure gauge 54 mounted on the wall of the chamber and responsive to the pressure within the chamber. The mercury will normally extend well into the chamber 54) and the pressure gauge 54 will indicate the pressure that is applied to this mercury. The pressure gauge will normally vary only slightly during operation of the heat exchanger and therefore any large variation in pressure whether it be an increase or a decrease in pressure will indicate improper operation requiring examination and inspection of the apparatus.

The legs of the U-tubes that make up the coils extend radially through the wall of the vessel and are secured to the vessel wall in a fluid tight manner by means of the annular connecting member 56 which is positioned over the outer tubular element 40 of the U-tube and is secured to the element by weld 58 and to the vessel wall by weld 60. This annular connecting member is so constructed as to accommodate limited relative movement between the ends of the U-tube extending through the vessel wall and the vessel wall which may be due to differential thermal expansion.

The heating fluid, such as liquid sodium, is supplied to the inner tubular element 38 of the U-tubes of coils 18 through inlet header 62 which is vertically mounted on vessel 10 in spaced relation to the wall of the vessel and is interconnected with the tubular element by means of connecting spurs 64 which are welded to the element 38 and which are formed in such a manner to accommodate their own thermal expansion without excessively stressing their connections with the header or the tubular element with this being accomplished by merely forming these spurs with expansion loops. The liquid sodium after passing through the tubular coils 18 is collected in the outlet header 68 which is of similar construction to inlet header 62 and is connected with tubular elements 38 by means of connecting spurs 70 which are similar in construction to connecting spurs 64.

In order to insure that the fluid that flows through vessel 10 is directed over the tube coils and does not flow through the relatively large areas 72 at the center of the coil these areas are lined with metallic walls 74 with bafile plate 76 being provided at the upper and lower end of these walls in covering relation with these areas so as to prevent fluid fiow therethrough and force the fluid to flow through the coils, which of course have their tubes spaced sufficiently to permit such flow.

In operation of the heat exchanger of this invention when the same is being employed as a steam generator, water is forced into drum 10 through opening 12 upwardly through the tube bundle 16 and out through opening 14 at the upper end of the vessel. Liquid sodium enters header 62 and is distributed to the various tubes that make up the tubular coils of the tube bundle with the sodium passing through these tubes and then into header 68 from which it is conveyed to a desired point. In passing through tube bundle 16 the water picks up heat from the sodium within the tubes of the bundle with this heat being transferred through the wall of tubular element 38, through the mercury in annular chamber 42 and then through the wall of tubular element 40 to the water which surrounds this latter tubular element. This heat is sufficient to evaporate at least a portion of this water and this steam and water mixture which leaves vessel 10 through opening 14 may be conveyed to a separating drum located above vessel 10 where the steam and water mixture is separated and the water returned to the vessel through opening 12 with a gravity circulation of this water being thus established. v

With the particular construction of tubular coils 18 herein disclosed excessive stresses at the connection of inner and outer tubular elements 38 and 40 have been completely eliminated and this is so even though the inner tubular element 38 has a higher coeflicient of expansion than the outer tubular element 40 because it must necessarily be fabricated of stainless steel having a high chromium-nickel content (such as 18-8) while the outer tubular element 40 is fabricated of ordinary carbon steel with no alloying elements. This prevention of excessive stresses wherein the inner and outer walls of double walled tubes are joined together is one of the most diflicult problems in designing a heat exchanger of this type because the wall that is in contact with the heating fluid is of course at a higher temperature than the wall that is in contact with the fluid to be' heated whereby the former wall is required to expand a greater amount than the latter wall, and with sodium being employedas the heating medium, necessitating the use of stainless steel in the wall that contacts this sodium, this situation is further aggravated. However, by forming the tube coils in the manner disclosed, i. e., of U-tubes wound into a spiral with the return bend at the center thereof, there are no straight tube portions of any substantial length in the coil which, it has been found, present difliculty with regard to stresses at these connections, and these coils have proved entirely free of objectionable stresses in this regard.

It will be further noted that informing the coil in this manner the length of the individual tubular elements that make up the tubular coil are'relatively short while the diameter of the coils themselves are relatively large thus enabling a large amount of surface to be packed into a vessel of relatively large diameter and small height with out developing, excessive pressure drops in the tubes in the coils. In this regard it will be noted that each of the coils consists of a pair of U-tubes that are wound into a spiral with this pair being in parallel flow and thereby requiring each of the tubes to be half the length that would be reguired if but a single U-tube were utilized to fabricate each coil.

In order that the various connections of the ends of the U-tubes with the vessel wall aswell as the connections of the connecting spurs 64 and 7t) may be accessible the inlet connections of the two adjacent U-tubes in each coil are laterally or circumferentially offset from each other and the outlet connections of these U-tubes are likewise laterally or circumferentially spaced from each other with the corresponding connection of the two adjacent coils also being circumferentially oflset so that these connections of adjacent coils are not located directly above each other. This requires that the inlet connections to the various U-tubes and the outlet. connections be distributed over an arc of the circumference of the vessel which is of substantial length and as is evident from Fig. 2 these inlet connections are distributed over the arc identified by the quarter sector 78 while the outlet'connections are distributed over the arc ofthe adjacent quarter'sector 80. Because of the numerous openingsthus provided in the drum half defined by these two adjacent quarter sectors it is necessary that the wall of this portion of the drum be of increased thickness in order that the necessary strength requirement of the drum will be maintained.

While we have illustrated and described a preferred embodiment of our novel organization it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the in vention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What we claim is:

1. A steam generator comprising a vertically disposed cylindrical vessel having an opening in the bottom thereof for the ingress of water thereinto and an opening in the top thereof for the egress of a steam and water mixture therefrom, a plurality of superimposed spiral tubular coils disposed within said vessel with the axis of said coils parallel with that of the vessel and with the diameter for each coil occupying substantially the full transverse section of the vessel, each of said coils comprising a U-tube wound in a spiral with the return bend at the center and with the legs thereof in side by side relation with the ends extending radially through the vessel wall in a fluid tight manner, means for passing hot liquid sodium through said tubular coils, the U-tubes of said coils being double walled and formed by a pair of concentric steel tubes that are interconnected at their ends in a fluid tight manner and form an annular chamber therebetween that is filled with L6 mercury with the inner of said concentric tubes that is in contact with the sodium being an alloy higherdnbhromium-nickel content thanthe outer *ofsaid'tubes which is in' contact with the water whereby said inner tube has a substantially greater coefficient of thermalcxpansion than said outer tube.

2. A heat exchanger for the' indirect transfer of heat from a heating fluid to afluid to be heated comprising a cylindrical vessel through which one of said fluids is adapted to flow, a plurality of spiral tube coils disposed Within said vessel and adapted to have the other-fluid flow therethrough, said coils beingadjacently disposed and having their axes parallel with the axis of the vessel with the diameter of each coil occupyingsubstantially the full cross section of thevessel, each coii including a pair of concentric tubes forming an annularchamber therebetween which is filled with a fluid having a high heat transfer coefficient, the ends of the outer of said tubes extending through and connected to the wall of the vessel in a fluid tight manner and the corresponding ends of the inner of said tubes being connected to the ends of said outer tube in a fluid tight manner, manifold means for supplying said other fluid tooneend of said coils and collecting said fluid from the other end of said coils.

3. In a heat exchanger-of the type described the combination of a vessel adapted to have a fluid flow therethrough, a plurality of spiral coils disposed within said vessel in spaced relation longitudinally of the vessel and with their axes generally parallel with the axis of the vessel, each of said coils including a U-tube wound into a spiral with the return bend at the center thereof and with the ends extending through the vessel wallw'ithsaid' U-tube comprising two concentric tubes forming an annular space therebetween which is filled with a fluid having a high heat transfer coeflicienn the ends of the outer of said tubes being secured to the vessel wall in a fluid tight manner and theends of the inner of said tubes being secured to the outer of said tubes in a fluid tight manner, and manifold means for supplying another fluid to one end of the inner of said tubes in each coil and collecting it from the other end.

4. The organization of claim 3=Wherein the two ends of each U-tube extend through the vessel wall at circumferentially spaced locations and wherein the locationsat which the corresponding ends of adjacent coils extend through the vessel wall are circumferentially offset.

5. A heat exchanger for the indirect transfer of heat from one fluid to another comprising a vertically disposed cylindrical vessel adapted to have one of said fluids flow therethrough, a tube bundle secured within the vessel and including spiral tubular coils disposed one above the other with their axes generally parallel with the axis of the vessel and with each coil sloping upwardly toward its center, each of said coils including a U-tube wound into a spiral with the return bend at the center thereof and with the ends extending through the vessel wall with said U-tube comprising two concentric tubes forming an annular space therebetween which is filled with a fluid having a high heat transfer coeflicient, the ends of the outer of said tubes being secured to the vessel wall in a fluid tight manner and the ends of the inner of said tubes being secured to the outer of said tubes in a fluid tight manner, and manifold means for supplying the other of said fluids to one end of the inner of said tubes in each coil and collecting it from the other end.

6. The organization of claim 5 wherein the two ends of each U-tube extend through the vessel wall at circumferentially spaced locations and wherein the locations at which the corresponding ends of adjacent coils extend through the vessel Wall are circumferentially offset.

7. A heat exchanger for the indirect transfer of heat from a heating fluid to a fluid to be heated comprising a vertically disposed cylindrical vessel adapted to have one of said fluids flow therethrough, a tube bundle secured within the vessel and comprising superimposed spiral tubular coils which have their axes generally parallel with the axis of the vessel and with each coil sloping upwardly toward its center, each of said coils including a pair of internested U-tubes wound into a spiral with the return bends at the center thereof and with the ends extending through the vessel wall in a fluid tight manner with each of said U-tubes comprising two concentric tubular members forming an annular space therebetween which is filled with a fluid having a high heat transfer coefficient, the ends of the inner of said tubes being secured to the ends of the outer of said tubes in a fluid tight manner, an inlet header connected to supply the inner of said tubes with the other of said fluids and an outlet header connected to receive said fluid from the other end.

8. A heat exchanger for the indirect transfer of heat from a heating fluid to a fluid to be heated comprising a vertically disposed cylindrical vessel through which one of said fluids is adapted to pass, a plurality of superimposed spiral tubular coils disposed within said vessel with the axis of the coils generally parallel with the axis of the vessel and with the diameter of each coil being such that the coil occupies substantially the full cross section of the vessel, each of said coils comprising a pair of tubular members in side by side relation and wound spirally inward with said members being reversely bent at the center of the spiral and then wound spirally outward with the outwardly spiralling portion disposed intermediate the inwardly spiralling portion, an inlet and an outlet header disposed outwardly of said vessel in circumferentially spaced relation and extending longitudinally of the vessel, the ends of each of said tubular members extending through the vessel wall in a fluid tight manner with the inlet end communicating with said inlet header and the outlet end communicating with said outlet header, each of said tubular members comprising a pair of concentric tubes welded together at their ends to form a closed annular chamber therebetween which is filled with mercury, the location at which the inlet ends of each of the tubular members extend through the vessel wall being within a given quarter sector of the vessels circumference with the inlet ends of the tubular members of adjacent coils being circumferentially offset from each other and the location at which the outlet ends of the tubular members extend through the vessel wall being within an adjacent quarter sector of the vessel circumference and similarly arranged with the semicircular sector of the vessel wall identified by said two quarter sectors being of increased thickness relative to the remainder of the vessel wall in order to compensate for the weakening thereof due to the numerous openings provided therein.

9. A heat exchanger for indirect transfer of heat from a heating fluid to a fluid to be heated comprising a vessel through which one of said fluids is adapted to flow, a plurality of spiral tubular coils disposed within said vessel with each coil comprising a U-tube Wound in the form of a spiral with the return bend at the center and with the legs thereof in side by side relation with the ends extending radially through the vessel wall in a fluid tight manner, said coils having their axes parallel with that of the vessel and being in adjacent relation to one another with the diameter thereof being such that each coil occupies substantially the full cross section of the vessel, means for passing the other of said fluid through said tubular coils, each of said U-tubes being double walled and formed by a pair of concentric tubes that are interconnected at their ends in a fluid tight manner and form an annular chamber therebetween that is filled with a fluid having a high heat transfer coeflicient with the metal of the concentric tube that is in contact with the heating medium having a substantially higher coeflicient of thermal expansion than the metal of the other of said concentric tubes.

10. The organization of claim 2 wherein the vessel is vertically disposed and has water conveyed upwardly therethrough and the coils slope upwardly toward their center and are supplied with hot liquid sodium.

11. The organization of claim 8, wherein the coils slope upwardly toward their center and are supplied with hot liquid sodium.

References Cited in the file of this patent UNITED STATES PATENTS 740,303 McDougall Sept. 29, 1903 1,439,988 Anderson Dec. 26, 1922 2,644,322 Preble et al. July 7, 1953 2,658,728 Evans Nov. 10, 1953 

